Implants are used in a variety of embodiments in modern medical technology. Implants serve, among other things, to support blood vessels, hollow organs and duct systems (endovascular implants), to attach and temporarily secure tissue implants and tissue transplants, and for orthopedic purposes, for example, as nails, plates or screws.
Thus, for example, implantation of stents has become established as one of the most effective therapeutic measures in the treatment of vascular diseases. The purpose of stents is to provide a supporting function in the hollow organs of a patient. Stents of a traditional design, therefore, have a filigree supporting structure of metallic struts, which are present initially in a compressed form for introduction into the body and then are widened at the site of application. One of the main areas of application of such stents is for permanently or temporarily widening vascular constrictions, in particular, constrictions (stenoses) of the myocardial vessels, and then keeping the constricted areas open. In addition, aneurysm stents are also known in the art that, for example, serve to support damaged vascular walls.
The basic body of each implant, in particular, of stents, comprises an implant material. For purposes of the present disclosure, an implant material is a non-living material that is used for application in medicine and which interacts with biological systems. The basic prerequisites for use of a material as an implant material, which is in contact with the body's environment when used as intended, is that the material must be compatible with the body (biocompatibility). For purposes of the present disclosure, biocompatibility is the ability of a material to induce an appropriate tissue reaction in a specific application. This includes an adaptation of the chemical, physical, biological and morphological surface properties of an implant to the recipient tissue to obtain a clinically desired interaction. The biocompatibility of the implant material also depends on the chronological course of the reaction of the biosystem in which the implant is located. Irritation and inflammation, which may lead to tissue changes, may thus occur relatively rapidly. Biological systems react in different ways depending on the properties of the implant material. Depending on the reaction of the biosystem, the implant materials may be subdivided into bioactive, bioinert and degradable/absorbable materials. For purposes of the present disclosure, only degradable/absorbable metallic implant materials are of interest. For purposes of the present disclosure, these degradable/absorbable metallic implant materials are referred to hereinbelow as biocorrodible metallic materials.
The use of biocorrodible metallic materials is recommended, in particular, because, in most cases, the implant need only remain temporarily in the body to fulfill the medical purpose. Implants of permanent materials, i.e., materials that are not degraded in the body, are optionally removed again because there may be rejection reactions on the part of the body in the medium range and in the long term, even when there is a high biocompatibility.
One approach to avoid an additional surgical procedure thus consists of forming the implant entirely or in part of a biocorrodible metallic material. For purposes of the present disclosure, biocorrosion refers to microbial processes or simply processes caused by the presence of endogenous media leading to a gradual degradation of the structure comprising the material. At a certain point in time, the implant or at least the part of the implant made of the biocorrodible material loses its mechanical integrity. The degradation products are largely absorbed by the body. As in the case of magnesium, for example, in the best case the degradation products even have a positive therapeutic effect on the surrounding tissue. Small quantities of unabsorbable alloy constituents are tolerable as long as they are nontoxic.
Known biocorrodible metallic materials include, but are not limited to, pure iron and biocorrodible alloys of the group consisting of the main elements of magnesium, iron, zinc, molybdenum and tungsten. It is proposed in German Patent Application No. 197 31 021 that, among other things, medical implants should be made of a metallic material whose main component is an element from the group consisting of alkali metals, alkaline earth metals, iron, zinc and aluminum. Alloys based on magnesium, iron and zinc are described as being especially suitable. Secondary constituents of the alloys may be manganese, cobalt, nickel, chromium, copper, cadmium, lead, tin, thorium, zirconium, silver, gold, palladium, platinum, silicon, calcium, lithium, aluminum, zinc and iron. In addition, German Patent Application No. 102 53 634 describes the use of a biocorrodible magnesium alloy containing >90% magnesium, 3.7-5.5% yttrium, 1.5-4.4% rare earth metals and <1% remainder, which are suitable, in particular, for the production of an endoprosthesis, e.g., in the form of a stent. Regardless of the advances that have been achieved in the field of biocorrodible metal alloys, the alloys known so far have only limited usability because of their corrosion behavior. The relatively rapid biocorrosion of the magnesium alloys, in particular, limits their field of use.
Traditional technical fields of use of molded bodies made of metallic materials, in particular, magnesium alloys, outside of medical technology usually require extensive suppression of corrosive processes. Accordingly, the purpose of most technical methods for improving corrosion performance is to completely inhibit corrosive processes. However, the goal of improving the corrosion performance of the biocorrodible metallic materials in the present disclosure lies not in complete suppression of corrosive processes but only in inhibition of corrosive processes. For this reason alone, most of the known measures for improving corrosion protection are not suitable. Furthermore, for a use in medical technology, toxicological aspects must also be taken into account. In addition, corrosive processes depend greatly on the medium in which the processes take place and, therefore, the findings about corrosion protection obtained under traditional environmental conditions in a technical (in vitro) environment should not be transferable to the processes in a physiological environment to an unlimited extent.
According to one approach of known technical methods for improving corrosion behavior (in the sense of increasing corrosion protection), a corrosion-preventing layer is produced on the molded body made of the metallic material. Known methods for creating a corrosion-preventing layer have been developed and optimized from the standpoint of technical use of the coated molded body, but not medical technical use in biocorrodible implants in a physiological environment. These known methods include, for example, applying polymers or inorganic cover layers, creating an enamel, chemical conversion of the surface, hot gas oxidation, anodizing, plasma sputtering, laser beam remelting, PVD methods, ion implantation or lacquering.
European Patent Application No. 0 993 308 describes a permanent stent coated by a PVD method with a carrier polymer to which perfluoroalkyl chains are bound. European Patent Application No. 0 560 849 describes an implant having a fluorinated polymer surface which is created by immersing the implant in a solution and then drying the implant. U.S. Pat. No. 5,246,451 discloses a plasma coating method for permanent vascular prostheses in which a polymer layer containing fluorine can be created by a plasma treatment. This polymer layer is then functionalized, again with the use of a plasma.
One feature of the present disclosure provides an improved or at least an alternative coating for an implant of a biocorrodible metallic material which produces a temporary inhibition but not complete suppression of corrosion of the material in a physiological environment.